The present invention relates to a process for producing microcapsules, more particularly, to a process for producing mlcrocapsules that have a sufficiently broad particle size distribution as to be suitable for use in a pressure measuring film.
One of the conditions that should preferably be satisfied by microcapsules suitable for use in a pressure measuring film is that they have a linear relationship between pressure and color density in order to provide pressure measurements of high precision. In order to provide a linear relationship between pressure and color density, the capsules must have a broad particle size distribution.
Conventionally, microcapsules prepared by a batch process of emulsification using a paddle blade or a dissolver blade are provided with a pressure-density curve of good linearity by blending two or more microcapsule solutions that have been produced by different methods (see U.S. Pat. No. 4,132,112) or by coating multiple layers of two or more microcapsules having different breaking strengths (see Unexamined Published Japanese Patent Application No. 78186/1979).
However, the microcapsules prepared by a batch process of emulsification using a paddle blade or a dissolver blade have only poor reproducibility of particle size distribution and suffer from the following disadvantages:
(1) preliminary coating and evaluation procedures are necessary for checking the performance of capsules, and this causes a loss in the capsule solution or film, potentially leading to a lower operational output; PA1 (2) preparing and blending two or more microcapsule solutions is a great operating load; and PA1 (3) the batch process of emulsification using a paddle blade or a dissolver blade produces not only coarse particles which can cause "color peppers" (spotty color formation) but also minute particles which in no way participate in color formation.
Therefore, in order to produce a product that is useful as a pressure measuring film, a filtering operation is necessary for rejecting any coarse particles present, but this is another factor that increases the operating load.
The present invention has been accomplished under these circumstances, and has an object the provision of a process for producing microcapsules that not only eliminates the possible loss of capsule solutions or films or operating efficiency of the coating applicator due to a process of checking the performance of capsules, but also obviates the need to perform blending and filtering operations.